1. Field of the Invention
The present invention relates to optical fiber cables and, more particularly, to optical fiber cables having a monotube design.
2. Description of the Prior Art
Optical fiber cables have been used for the past several years to transmit information at high rates and long distances. The transmission medium is composed of hair-thin optical fibers which are protected from external forces by precisely designed and manufactured cable structures. Cable structure families which are currently being used are:
A. Loose Tube: Structure in which several gel filled buffer tubes containing optical fibers are stranded around a central strength member.
B. Slotted Core: Structure in which optical fibers are precisely placed in gel filled channels or slots. The channels are symmetrical and form a helical path along the longitudinal axis of the cable. A strength member is located in center of the cable structure.
C. Monotube: Structure in which all of the optical fibers are in a single, centrally located, gel filled buffer tube.
All of the cable structures listed above also incorporate additional protection which may consist of radially applied strength members, corrugated armor, and plastic sheaths.
Two monotube cable structures are currently being manufactured by AT&T (LXE design) and Siecor (Maxitube design). Cable cross-sectional views of the AT&T LXE design generally indicated by the numeral 10 and Siecor Maxitube design generally indicated by the numeral 30 can be found in FIGS. 1 and 2, respectively.
The AT&T LXE cable 10 consists of a single large plastic, gel filled buffer tube 11 which can contain up to 96 optical fibers 12. The buffer tube 11 is surrounded by water-swellable tape 14 and corrugated armor 16. A ripcord 18 is placed under the corrugated armor 16 to aid in sheath removal. A strip of water-swellable tape 15 is helically wrapped around the corrugated armor. Two steel strength members 20 are located 180 degrees apart outside the corrugated armor 16. The armor 16 and strength members 20 are encapsulated by a high-density polyethylene jacket 22 which bonds to the armor 16 and completes the structure.
The cable 10 has several weaknesses in design, processibility and installation. First, the two steel strength members 20 with outer diameters of 0.063" produce an extremely stiff cable. Secondly, the steel strength members have a tendency to "piston" during installation and termination. The term "pistoning" describes the longitudinal movement of the steel strength members relative to the other cable components. Thirdly, the sheathing process is more complex due to the steel strength members 20 impregnated in the jacket 22. Fourthly, water penetration susceptibility exists between the steel strength members 20 and armor 16, and in areas void of water-swellable tape 15. Finally, the non-metallic AT&T LXE cable design is significantly more expensive to produce than the Siecor Maxitube design.
The Siecor Maxitube cable 30 consists of a single, large, gel filled, dual-layer buffer tube 32 which can contain up to 12 optical fibers 34. The buffer tube 32 is surrounded by radial strength yarns 36 which are impregnated with filling compound. Flooded, corrugated armor 38 is applied over the radial strength yarns 36. A ripcord 40 is placed over the corrugated armor 38 to aid in the removal of the outer jacket 42. A medium-density polyethylene jacket 42 is applied over the armor to complete the structure.
The cable 30 also has a number of drawbacks. First, the buffer tube 32 contains up to 0.6% fiber overlength, which is difficult to process in buffering. Secondly, the cable 30 has the capability of containing only 12 fibers. Thirdly, the buffer tube filling compound has a drip susceptibility at 65.degree. C. due to low viscosity. Fourthly, cable flexibility causes reduced sheave spacing during an aerial installation. Fifthly, the two-layer buffer tube 32 is difficult to process. Finally, taunt mid-span entry of the cable 30 is difficult.